This invention relates generally to the production of extreme ultraviolet and soft x-rays with an electric discharge source for projection lithography.
The present state-of-the-art for Very Large Scale Integration (xe2x80x9cVLSIxe2x80x9d) involves chips with circuitry built to design rules of 0.25 xcexcm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (xe2x80x9cUVxe2x80x9d) delineating radiation. xe2x80x9cDeep UVxe2x80x9d (wavelength range of xcex=0.3 xcexcm to 0.1 xcexcm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 xcexcm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require x-ray wavelengths. Various x-ray radiation sources are under consideration. One source, the electron storage ring synchrotron, has been used for many years and is at an advanced stage of development. Synchrotrons are particularly promising sources of x-rays for lithography because they provide very stable and defined sources of x-rays, however, synchrotrons are massive and expensive to construct. They are cost effective only when serving several steppers.
Another source is the laser plasma source (LPS), which depends upon a high power, pulsed laser (e.g., a yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d) laser), or an excimer laser, delivering 500 to 1,000 watts of power to a 50 xcexcm to 250 xcexcm spot, thereby heating a source material to, for example, 250,000xc2x0 C., to emit x-ray radiation from the resulting plasma. LPS is compact, and may be dedicated to a single production line (so that malfunction does not close down the entire plant). The plasma is produced by a high-power, pulsed laser that is focused on a metal surface or in a gas jet. (See, Kubiak et al., U.S. Pat. No. 5,577,092 for a LPS design.)
Discharge plasma sources have been proposed for photolithography. Capillary discharge sources have the potential advantages that they can be simpler in design than both synchrotrons and LPS""s, and that they are far more cost effective. Klosner et al., xe2x80x9cIntense plasma discharge source at 13.5 nm for extreme-ultraviolet lithography,xe2x80x9d Opt. Lett. 22, 34 (1997), reported an intense lithium discharge plasma source created within a lithium hydride (LiH) capillary in which doubly ionized lithium is the radiating species. The source generated narrow-band EUV emission at 13.5 nm from the 2-1 transition in the hydrogen-like lithium ions. However, the source suffered from a short lifetime (approximately 25-50 shots) owing to breakage of the LiH capillary.
Another source is the pulsed capillary discharge source described in Silfvast, U.S. Pat. No. 5,499,282, which promised to be significantly less expensive and far more efficient than the laser plasma source. However, the discharge source also ejects debris that is eroded from the capillary bore and electrodes. An improved version of the capillary discharge source covering operating conditions for the pulsed capillary discharge lamp that purportedly mitigated against capillary bore erosion is described in Silfvast, U.S. Pat. No. 6,031,241.
Debris generation remains one of the most significant impediment to the successful development of the capillary plasma discharge sources in photolithography. Ultimately, this will reduce their efficiency to a point where they must to be replaced more often than is economically feasible. The art is in search of capillary plasma discharge sources that do not generate significant amounts of debris.
The present invention is based in part on the demonstration that debris generation within the electric capillary discharge source is dependent on the magnitude and profile of the electric field that is established along the surfaces of the electrodes. An electrode shape that results in uniform electric field strength along its surface will minimize sputtering and debris generation. Electrostatic models have been developed to predict the electric field between the electrodes. These models were applied to optimize the shape of the electrodes. An iterative approach was used in the optimization process. The predicted electric field strength profiles along the surface of the optimized electrodes show a lower peak value and improved uniformity over the initial electrode designs.
Accordingly, in one embodiment the invention is directed to an extreme ultraviolet and soft x-ray radiation electric discharge plasma source that includes:
(a) a body that defines a circular capillary bore that has a proximal end and a distal end;
(b) a back electrode positioned around and adjacent to the distal end of the capillary bore wherein the back electrode has a channel that is in communication with the distal end and that is defined by a non-uniform inner surface which exhibits a first region which is convex, a second region which is concave, and a third region which is convex wherein the regions are viewed outwardly from the inner surface of the channel that is adjacent the distal end of the capillary bore so that the first region is closest to the distal end;
(c) a front electrode positioned around and adjacent to the proximal end of the capillary bore wherein the front electrode has an opening that is communication with the proximal end and that is defined by a non-uniform inner surface which exhibits a first region which is convex, a second region which is substantially linear, and third region which is convex wherein the regions are viewed outwardly from the inner surface of the opening that is adjacent the proximal end of the capillary bore so that the first region is closest to the proximal end; and
(d) a source of electric potential that is connected across the front and back electrodes.
In another embodiment, the invention is directed to a method of producing extreme ultra-violet and soft x-ray radiation that includes the steps of:
(a) providing an electric discharge plasma source that comprises:
(i) a body that defines a circular capillary bore that has a proximal end and a distal end;
(ii) a back electrode positioned around and adjacent to the distal end of the capillary bore wherein the back electrode has a channel that is in communication with the distal end and that is defined by a non-uniform inner surface which exhibits a first region which is convex, a second region which is concave, and a third region which is convex wherein the regions are viewed outwardly from the inner surface of the channel that is adjacent the distal end of the capillary bore so that the first region is closest to the distal end;
(iii) a front electrode positioned around and adjacent to the proximal end of the capillary bore wherein the front electrode has an opening that is communication with the proximal end and that is defined by a non-uniform inner surface which exhibits a first region which is convex, a second region which is substantially linear, and third region which is convex wherein the regions are viewed outwardly from the inner surface of the opening that is adjacent the proximal end of the capillary bore so that the first region is closest to the proximal end; and
(iv) a source of electric potential that is connected across the front and back electrodes; and
(v) a housing that defines a vacuum chamber that is in communication with the opening of the front electrode;
(b) introducing gas from a source of gas into the channel of the back electrode and into the capillary bore; and
(c) causing an electric discharge in the capillary bore sufficient to create a plasma within the capillary bore thereby producing radiation of a selected wavelength less than about 1xc3x9710xe2x88x923 Torr.
An important feature of the invention is with the design of the front and back electrodes, the source of electric potential establishes substantially uniform first electric fields along the inner surface of the front electrode and substantially uniform second electric fields along the inner surface of the back electrode.